Fuel cells directly convert chemical energy of a fuel into electricity. Recent development is directed to improving the performance of SOFCs because these fuel cells are able to convert a wide variety of fuels with a high efficiency.
A single SOFC comprises a solid oxide dense electrolyte sandwiched between an anode (fuel electrode) and a cathode (oxygen electrode), said anode and cathode each having fine pores or channels for supplying the reactants. Upon passing an oxygen-containing gas such as air along the cathode, the oxygen molecules contact the cathode/electrolyte interface where they are electrochemically reduced to oxygen ions. These ions diffuse into the electrolyte material and migrate towards the anode where they electrochemically oxidize the fuel at the anode/electrolyte interface. The electrochemical reactions within the fuel cell provide electricity for an external circuit. The fuel cell may further comprise a support having fine pores or channels, which enable the controlled distribution of the fuel. A plurality of SOFCs may be connected in series via interconnects to form a so-called “SOFC stack”.
A SOFC may be operated reverse, i.e., as an electrolysis cell (SOEC), which directly converts electricity into chemical energy of a fuel. For example, the electrochemical decomposition of steam leads to hydrogen and oxygen, or the electrochemical decomposition of carbon dioxide leads to carbon monoxide and oxygen. This means that the electrolysis of a mixture of steam and carbon dioxide leads to a mixture of hydrogen and carbon monoxide (syngas), which in turn can be converted into fuels such as methanol or dimethyl ether (DME) using well-known process technologies. SOECs have the potential of efficiently converting renewable energies such as wind energy, photovoltaic energy or hydropower. Recent developments are directed to reversible solid oxide cells (SOCs), which may be used both as SOFC and SOEC.
The anode and the cathode of a SOFC are made from materials having electrical conductivity but no ion conductivity, whereas the electrolyte thereof is made from a material having ion conductivity but no electrical conductivity.
Suitable materials for the cathode, the electrolyte, and the anode of a SOFC are known in the art (see, for example, U.S. Pat. No. 7,498,095 and WO-A-01/43524). A commonly used cathode material is lanthanum strontium manganite (LSM), a cermet such as yttria stabilized zirconia (YSZ), or a composite thereof. The anode material is generally a cermet such as YSZ. If hydrogen is used as the fuel it is electrochemically oxidized by the oxygen ions at the anode/electrolyte interface. In case a hydrocarbon such as methane is to be used as the fuel, a reforming catalyst such as nickel is added to the anode material. The catalyst assists in converting the fuel into hydrogen, known as internal reforming. The solid oxide electrolyte material is generally a ceramic material such as YSZ, which exhibits sufficient ion conductivity only at high temperatures. Therefore, a SOFC has to be operated at an elevated temperature (usually at least 300° C.) in order to achieve a high current density and power output.
An anode comprising a reforming catalyst is not resistant to oxygen while the SOFC is warmed up to above a certain temperature, i.e., about 200° C. Oxygen at elevated temperatures can damage the anode reforming catalyst due to low redox stability. Therefore, the SOFC stack must be protected against an oxidizing gas during start up and shut down.
WO-A-2008/001119 discloses the use of a blanket gas based on an inert gas during shut down of a SOFC stack. This publication further discloses the use of a catalytic partial oxidation reactor to produce reducing gas, which comprises carbon monoxide and hydrogen, in order to protect a SOFC stack against an oxidizing gas during start up and shut down.
One advantage of reducing gas is that only anode channels need to be purged while air on the cathode side can be used for heating up or cooling down during start up and shut down, respectively. Hydrogen can basically protect nickel particles of the anode surface to react with oxygen ions, which may be transferred from the cathode to the anode via the electrolyte.
However, reducing gas has two major drawbacks. First, hydrogen and carbon monoxide cannot be purged into the atmosphere. The catalytic burner of a SOFC system needs to be operated during start up and shut down of the fuel cell to burn the toxic and explosive gases. This may cause several difficulties regarding the process operability and safety.
Second, carbon monoxide at low temperatures (usually less than 300° C.) may react with nickel nano-particles on the anode surface and on the pre-reforming/reforming catalyst, which leads to the formation of nickel carbonyl (Ni(CO)4; boiling point 43° C.). Nickel carbonyl is highly volatile and extremely toxic. Even a low concentration of this compound in the air can be fatal (LC50=3 ppm). Moreover, nickel carbonyl may be thermally decomposed inside the catalytic burner, which leads to a deactivation of the catalyst.
On the other hand, protection gas can be easily purged into the atmosphere and has no interaction with catalyst and fuel cell materials. However, it cannot protect the reforming catalyst at the anode surface against oxygen ions, which may diffuse through the solid electrolyte from the cathode side. This problem is more serious at high temperatures close to the operating temperature of the fuel cell where the ion conductivity of the electrolyte is high.
Therefore, protection gas may be used when the SOFC stack temperature is below 300° C. while the catalytic burner may not be operating. Normally, above this temperature, the catalytic burner temperature in the system is beyond the minimum operating temperature; therefore, reducing gas can be used to protect the SOFC stack. Since the reducing gas has a low carbon monoxide content, there is less chance for carbon formation.
The above-described problems also arise during start up and shut down of a SOEC.